1. Field of the Invention
The present invention relates to a light emitting device, particularly to the light emitting device in which a plurality of light emitting diode (LED) chips is connected in series to each other so as to form an LED string, a plurality of LED strings is connected in parallel to each other, and an ultraviolet light LED chip or a violet LED chip and a blue LED chip are mixed.
2. Description of the Related Art
JP-A-2014-143307 discloses a light emitting module which includes a plurality of long light emitting portions arranged on a board. In each of the plurality of light emitting portions, a plurality of sets of light emitting portions which have different color temperatures and are adjacent to each other is included. In order to cause the light emitting portions to have different color temperatures, a ratio of phosphors for colors mixed in a wavelength conversion member included in one light emitting portion is caused to be different from that in another light emitting portion.
JP-A-2005-136006 discloses a light emitting device including an ultraviolet light LED chip and a light color conversion member which converts ultraviolet rays emitted by the ultraviolet light LED chip into white light. In JP-A-2005-136006, the light emitting device further includes visible LED chips which emit light in the visible range, in addition to the ultraviolet light LED chip. The visible LED chips are at least one of a blue LED, a green LED, and a red LED.
If white clothes are irradiated with ultraviolet light, an effect of causing the white color of the clothes to be shown more beautifully is obtained. However, as disclosed in JP-A-2005-136006, in the light emitting device having a combination in which the ultraviolet light LED chip and the light color conversion member are combined, since visualization of the ultraviolet light LED chip can hardly be sensed, there is a defect in that effective use of leaked ultraviolet light as the visible light is impossible and luminous efficiency is degraded. Meanwhile, a light emitting device including a blue LED chip and a light color conversion member which converts blue light emitted by the blue LED chip into white light has been used more widely than before. However, in the light emitting device having a combination of the blue LED chip and the light color conversion member, since there is an advantage of high luminous efficiency, but the application of ultraviolet light is impossible, there is a defect in that the effect of causing the white color of clothes to be shown beautifully is not obtained. Thus, if the blue LED chip is included in addition to the ultraviolet light LED chip, as disclosed in JP-A-2005-136006, it is possible to obtain the effect of causing the white color of clothes to be shown beautifully, and to improve luminous efficiency.
Generally, the threshold voltage (Vf) of the ultraviolet light LED chip has a voltage value higher than the threshold voltage of the blue LED chip. For example, the threshold voltage of the ultraviolet light LED chip may have a range formed by the minimum value of 3.1 V, the reference value of 3.4 V, and the maximum value of 3.7 V. On the contrary, the threshold voltage of the blue LED chip may have a range formed by the minimum value of 2.8 V, the reference value of 3.0 V, and the maximum value of 3.2 V.
A method in which a plurality of LED chips is connected in series to each other so as to form an LED string (LED series circuit) and a plurality of LED strings is connected in parallel to each other is used in order to obtain the sufficient quantity of light from the light emitting device using LED chips. Here, when each of the LED strings is configured only by the ultraviolet light LED chip or the blue LED chip, the synthetic threshold voltage of the LED string (summation value of the threshold voltages of LED chips connected in series to each other) has a voltage value which is different for each of the LED strings. Thus, since a current flowing in the LED string (LED string configured only by the blue LED chip) having a low synthetic threshold voltage is greater than a current flowing in the LED string (LED string configured only by the ultraviolet light LED chip) having a high synthetic threshold voltage, and light emitting intensity of one LED string is different from that of another LED string, there is a problem in that light emitting unevenness occurs between the LED strings. This problem is also similarly caused when the ultraviolet light LED chip is substituted with the violet LED chip.
In JP-A-2014-143307, only a technology in which a plurality of LED chips of any one type among the visible LED chip of colors and the ultraviolet light LED chip is connected in series and parallel to each other is disclosed, and a technology in which ultraviolet light LED chips and blue LED chips are mixed and connected in series and parallel to each other is not disclosed at all. In JP-A-2005-136006, only a technology in which the visible LED chips for the colors and the ultraviolet light LED chips are mixed is disclosed, and a specific connection structure of the LED chips is not disclosed at all.